This invention relates to an automatic forms compensator for sensing the thicknesses of various media mounted on a platen of a serial type printer, and for maintaining the print head thereof, a predetermined distance way from the particular medium being printed upon.
One variety of a prior art wire matrix printer, which is designed to produce characters while using the "5 by 7" matrix format, utilizes a print head in which seven print head wires are arranged in a vertical column, and the print head is indexed serially along a line of printing on the platen associated with the printer. Actuation of the seven print head wires in one "impact" is all that is necessary to complete a simple character like the numeral "1", for example; however, to produce a more complicated character like the letter "M" up to five successive "indexes" and five selective "impacts" by the print head wires may be necessary to complete the printing of the character while the print head is traversed along the line of printing. In order to obtain a fast rate of printing, it is necessary that the associated hammers which drive the matrix print head wires operate with a very short stroke. The short stroke of the print head wires makes a printer employing this type of printing mechanism of limited value in accounting machines, for example, because for this type of machine, it is necessary that the printing mechanism be capable of printing on a variety of thicknesses of media. It is necessary that the print mechanism for an accounting machine print on a tally or audit roll which has a complete record of all transactions and also print on a pass or bank book. Obviously, a bank book is generally much thickner than an audit roll, and this wide range of thicknesses of media makes it extremely difficult for a wire matrix printer with a short stroke drive mechanism to print satisfactorily when media of such varying thicknesses are mounted on the same platen. If the platen is adjusted backward to accommodate the thick pass book, for example, the short stroke drive mechanism will not be able to impact the matrix print head wires against the thinner audit roll, and if the platen is adjusted forward for printing upon the thinner audit roll, it is possible that there might be insufficient clearance between the print head and the pass book to permit the print head to pass thereover, and jamming of the print head against the edge of the pass book will result.
The present invention eliminates the problems cited in the previous paragraph, in that the forms compensator disclosed herein will enable the print mechanism of an impact printer (like a wire matrix printer) to print satisfactorily on media of varying thicknesses mounted on a platen, automatically, without any attention from an operator of the machine. In prior art printers, it was necessary for an operator to adjust the platen either forward or backward, depending upon the thickness of the media inserted thereon.
Another advantage of the present invention is that a sensing means which is employed in the automatic forms compensator to sense the thickness of the media to be printed upon, is also utilized for lightly compressing multi-ply forms so as to enable a larger number of clearer copies to be produced by a printer employing this invention than is attainable by a printer of the prior art.
Another feature of the present invention is that the forms compensator permits the associated print head to be retracted to permit visual inspection of a line being printed. The retraction of the print head can be effected either manually or mechanically.